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Date:   Tue, 5 Jan 2021 17:53:18 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Michael Kelley <mikelley@microsoft.com>
Cc:     Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        vkuznets <vkuznets@redhat.com>,
        "jwiesner@suse.com" <jwiesner@suse.com>,
        "ohering@suse.com" <ohering@suse.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>
Subject: Re: [PATCH v2] x86/hyperv: Fix kexec panic/hang issues
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On Tue, Jan 05, 2021 at 04:39:38PM +0000, Michael Kelley wrote:
> From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, January 5, 2021 5:04 AM
> > 
> > On Mon, Dec 21, 2020 at 10:55:41PM -0800, Dexuan Cui wrote:
> > > Currently the kexec kernel can panic or hang due to 2 causes:
> > >
> > > 1) hv_cpu_die() is not called upon kexec, so the hypervisor corrupts the
> > > old VP Assist Pages when the kexec kernel runs. The same issue is fixed
> > > for hibernation in commit 421f090c819d ("x86/hyperv: Suspend/resume the
> > > VP assist page for hibernation"). Now fix it for kexec.
> > >
> > > 2) hyperv_cleanup() is called too early. In the kexec path, the other CPUs
> > > are stopped in hv_machine_shutdown() -> native_machine_shutdown(), so
> > > between hv_kexec_handler() and native_machine_shutdown(), the other CPUs
> > > can still try to access the hypercall page and cause panic. The workaround
> > > "hv_hypercall_pg = NULL;" in hyperv_cleanup() is unreliabe. Move
> > > hyperv_cleanup() to a better place.
> > >
> > > Signed-off-by: Dexuan Cui <decui@microsoft.com>
> > 
> > The code looks a bit intrusive. On the other hand, this does sound like
> > something needs backporting for older stable kernels.
> > 
> > On a more practical note, I need to decide whether to take it via
> > hyperv-fixes or hyperv-next. What do you think?
> > 
> 
> I'd like to see this in hyperv-fixes and backported to older stable kernels.
> In its current form, the kexec path in a Hyper-V guest has multiple problems
> that make it unreliable, so the downside risk of taking these fixes is minimal
> while the upside benefit is considerable.

Applied to hyperv-fixes.

Wei.